The conducting channel in a pentacene organic thin-film transistor (OTFT) under light illumination was visualized by using a near-field scanning microwave microscope (NSMM). The increase in conductivity due to the photogeneration effect was caused by generation and accumulation of excess holes in the pentacene channel near the source region. As the light intensity increased, the threshold voltage was decreased due to the photogeneration of carriers through the pentacene layer. The photocurrent spectrum correlates with the optical absorption spectrum; the maximum value for conductivity was obtained at the wavelength with the highest optical absorption. The visualization of conductivity–voltage behavior provides a detailed configuration of the carrier injection, the mobile charge distribution, the linear range and the saturation beyond pinch-off in the transistor channel and directly evaluates the injection charge and the mobile charge. Thus, the present combination of NSMM images with the current–voltage characteristics is very effective for evaluating the carrier distribution along the channel and the boundary of the exhaustion region due to injection and photogeneration. The NSMM provides a unique approach to investigate the photogeneration transport mechanism along the conducting channel in OTFT.